[MapProxy] Strange WMS lookalike

[A.Pirard.Papou]

Hi,

I'm using Mapproxy 1.4.0 and I'm facing a strange (to me) server.
The UI is an "ArcGIS Server REST API" at geoportail.wallonie.be/walonmap
and it makes  WMS-like but not-WMS requests like this:
http://geoservices.wallonie.be/arcgis/rest/services/TOPOGRAPHIE/PICC/MapServer/export?dpi=96&transparent=true&format=png8&bbox=237697.72401485278%2C135612.54079507195%2C240495.69836080144%2C137632.63858526756&bboxSR=31370&imageSR=31370&size=705%2C509&_ts=1362703550248&f=image
With a plain WMS source config, Mapproxy sends a plain WMS GetMap 
request and is answered with a HTML page.

What is the Mapproxy configuration to access it?
Preferably with cache and layer configs to be sure.
What standard is this and where is it documented?

Thanks!

Cheers,

André.


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